The distance between optical terminals of optical fibre transmission systems is limited by various factors, including the optical power of the optical signal being transmitted. If the power is too low, the signal will be difficult to distinguish from noise. If the power is too great, distortion of the signal will occur. One type of distortion which can occur in optical fibre is self-phase modulation. The onset of this type of non-linear degradation can be quite sharp, in that only one or two dB of increase in power level can push a signal from optimal performance to a failed state.
Thus, for a given target acceptable bit error rate (BER), or acceptable risk of any errors, there will be an upper limit to the optical power of the optical signal when it is launched into an optical fibre by a transmitter. Correspondingly, there will be a lower power limit or threshold for the optical signal at the receiver, following inevitable attenuation in the optical fibre. Output power levels generally need to be held as high as possible so that the power level after attenuation by the optical link does not fall below the noise threshold, and become excessively degraded by optical noise.
Careful control of the output power of transmitters and of repeaters, or optical amplifiers is therefore necessary. Other optical elements such as filters, attenuators, dispersion compensators, and so on may also need to be controlled bearing in mind the optical signal power requirements. The power gains and losses in the optical path, and in the various optical elements in the path, will vary with wavelength, age, and temperature. In some cases, the power will be affected by multiplexed signals being added or removed. For example, in optical amplifiers, the gain at each wavelength depends upon the pump power into the amplifier and on the number and power levels of the signals present.
Conventionally, to ensure that all signals in the transmission system do not suffer excessive degradation, the worst case sums of all these variations must be identified for a particular system, and a margin of error, a power margin, must be allowed for the worst case variations. This margin reduces the available performance of the system, for example reducing the maximum allowed transmission distances between repeaters or optical amplifiers.
It is known to control optical amplifiers to maintain a constant pump current, or a constant pump power, or a constant gain. However, the preferred type of control is constant total output power control. The control of gain in an optical amplifier such as an erbium-doped amplifier is discussed in U.S. Pat. No. 5,088,095. Undesirable gain fluctuations resulting from saturation effects in the amplifier are compensated in a number of ways. A feed forward automatic gain control loop acts on the pump source to increase the gain when a transient of higher signal power is detected at the amplifier input. A second method is to compensate for any variation of signal input by actively counter modulating the optical power of one of the input channels. Finally, it is suggested to feed back a selected wavelength from the output to the input. Ring lasing occurs at the feedback wavelength and consequently the gain in the amplifier is held at a constant value.
It is known from U.S. Pat. No. 5,513,029 (Roberts) to measure the relative output powers of different wavelengths in a wavelength division multiplexed (WDM) system, and control the individual wavelength powers. This document is not considered to be prior art under USC 102 as it originates from the same inventor and was published on Apr. 30, 1996.
In many situations, it will not be practical to control separately the output powers of different wavelength bands in a WDM system. Where it is only practical to control the overall output power of an optical element, or the power of two or more bands, there remains the necessity to allow for considerable power margins. For example, when more than one optical wavelength is amplified by an optical amplifier, a desired total power level may be set at 20 milliwatts. For four wavelengths, this gives a mean power of 5 milliwatts per wavelength. However, owing to gain variations including gain tilt, one wavelength may be at 17 milliwatts and another three may be at 1 milliwatt each. The 17 milliwatt signal will be severely degraded by non-linearities. This example is over simplified for the purpose of clarity.